Your points are mostly valid but a few nitpicks:
"And even worse, it's completely cut off from the metro UI. While metro can intrude and present notifications on the desktop (which I find to be great), there is no vice versa."
This technically isn't true (though it might be true in practice, depending on what desktop apps you use) - desktop apps do have access to the APIs for the new notifications which can appear on top of Metro style apps and the start screen. However, support has to be added by the developer (so far the only app I'm aware of that supports them is Outlook).
" you'll find it very difficult to read e-mail and then quickly switch to another song in a music app while in Metro."
While I don't disagree with your general point, this particular scenario is pretty easy - you can just hit a volume control key or button (on a keyboard, tablet, or whatever) and an overlay will appear displaying the now-playing background audio, next, previous and pause/play controls.
"The start menu. Although it's fast, I see no reason for it to take the whole screen, and for it to be so drastically lacking in customization. I can't even drag a shortcut to it."
Technically you can put any desktop shortcut on the start screen (though not by dragging). You can right-click or use the ribbon for desktop apps/executables and folders, and though it's not exposed in the UI, it's actually possible to pin individual documents as well by copying shortcuts to them into the Start Menu folder (the same one in Windows 7) - you can get there via "Open file location" on the right-click menu for a pinned desktop app. This is basically a hack and for some reason it takes a few minutes for the document tile to show up.